1. Field of the Invention
The invention relates to an intake port structure of an internal combustion engine and more particularly to an intake port structure of an internal combustion engine that includes an airflow control valve which is pivotally supported at its proximal end in proximity to the inner wall of the intake port.
2. Description of the Related Art
A known intake port structure of an internal combustion engine includes an airflow control valve that produces vortical flows, such as tumble flows (longitudinal vortexes) and swirl flows (lateral vortexes). Such vortical flows facilitate mixing of fuel and air and help flames propagate within a combustion chamber, thereby improving the combustion efficiency. In addition, by using such vortical flows, it is possible to concentrate dense air-fuel mixtures around each spark plug and perform stratified combustions. Among such intake port structures, there is known an intake port structure in which an airflow control valve which is pivotally supported at its proximal end is fully opened by being retracted into the inner wall of the intake port, which is disclosed, for example, in Japanese Utility Model Application Publication No. 7-25264.
To allow the pivotal motion of the airflow control valve, it is necessary to provide, in each side of the airflow control valve, gaps (clearances) between the side ends of the airflow control valve and the inner walls of the intake port that face the same side ends of the airflow control valve. FIG. 7A to 7C schematically show a conventional intake port structure 100X of an internal combustion engine (hereinafter, simply referred to as an “intake port structure”) that includes an airflow control valve 1X which is pivotally supported at its proximal end on a valve shaft 2. More specifically, FIG. 7A shows the intake port structure 100X in a state where the airflow control valve 1X is fully closed, FIG. 7B shows the intake port structure 100X in a state where the airflow control valve 1X is fully opened, and FIG. 7C shows, for easier understanding of the shape of the airflow control valve 1X, the appearance of the airflow control valve 1X as well as the names of respective portions thereof.
A recess 11X is formed in the intake port 10X. The airflow control valve 1X is retracted into the recess 11X formed in the inner wall of the intake port 10X when the airflow control valve 1X is fully opened, as shown in FIG. 7B. As the recess 11X is thus formed in the intake port 10X, the recess 11X, the inner wall of the intake port 10X, and the wall substantially perpendicular to these recess and inner wall form a step. In the meantime, the gaps are formed between the side ends of the airflow control valve 1X and the inner wall of the intake port 10X. Therefore, when the airflow control valve 1X is fully closed, the intake air flows through these gaps as gap flow F2, in addition to main flow F1 that flows through the intentionally narrowed intake passage. Some gap flow F2, after passing through the gaps, collides with the step and thereby changes its direction upward, just like bouncing up at the step. This gap flow F2 may disturb the main flow F1, making it difficult to stably produce vortical flows in the combustion chamber.
In recent years, an ignition timing retardation control that retards the ignition timing of an internal combustion engine has been widely employed in order to quickly increase the temperature of a catalyst to a reaction temperature after the engine is started. However, when the ignition timing retardation is performed in an internal combustion engine that includes an airflow control valve for producing vortical flows, it is important whether it is possible to maintain a desired stability of combustion using vortical flows, in other words, whether it is possible to stably produce vortical flows, which is fundamentally necessary to achieve stable combustion. That is, unless vortical flows are stably produced, the ignition timing retardation is accordingly limited, and as a result, the effect of reducing emissions from the internal combustion engine is limited inevitably.